GB2029669A

GB2029669A - Moving coil transducers

Info

Publication number: GB2029669A
Application number: GB7928676A
Authority: GB
Original assignee: LIPSCHUTZ K
Current assignee: LIPSCHUTZ K
Priority date: 1978-05-18
Filing date: 1979-08-17
Publication date: 1980-03-19

Abstract

In a moving coil transducer, the centering forces acting upon its diaphragm 2 (Figure 1) (e.g. the mechanical suspension and the enclosed air in a sealed enclosure) are opposed by two coils 9A, 9B, or by one coil subdivided into two coils by a tap halfway along its length, the coils being so arranged that, when an adjustable D.C. is passed through them, forces are produced which act respectively upon the two coils and are balanced only when the diaphragm is at its mid position. These balancing coils may use the same air gap as the voice coils (Figure 3, not shown), or an air gap of a different magnet system (Figure 2, not shown). The balancing coils may be the voice coil itself (Figure 4, not shown). Alternatively the balancing coils may be mounted on a passive diaphragm. <IMAGE>

Description

SPECIFICATION Improvements in loudspeaker design Acoustic transducers, particularly loudspeakers, are often mounted in enclosures to prevent cancellation of the sound pressure waves produced by the front and rear of the transducer's diaphragm.

When a transducer is mounted in the wall of a sealed enclosure, the confined air is alternatingly compressed and rarefied as a result of the diaphragm movement acting like the piston in a pump.

These air pressure changes can be quite high especially when the enclosed air is of very small volume. The air pressure then acts like a spring developing a centering force for the diaphragm, adding its force to that of the transducer's diaphragm suspension. The combined effect not only causes the 'main system resonance' of a speaker to appear above most of the lower bass frequencies, resulting in a peak in the speaker's response curve, at d frequency where it causes distortion, but the combined and excessive centering force also impedes the wide swing of the diaphragm when needed at bass frequencies, i.e. below main system resonance, in order to produce sound output at a level comparable with output at other frequencies.In orderto maintain a musical balance it is therefore necessary to inject considerable bass boost, which may not readily be available, and is wasteful of amplifier power.

The present invention aims to overcome these disadvantages of existing transducers by making use of the fact that the reaction of the enclosed air on the movement of the diaphragm, i.e, its pressure change, is known, predictable, and always repeated for a given diaphragm movement. This known reaction on the diaphragm can therefore be balanced out by equal forces acting in the opposite direction. According to the main feature of the present invention, adjustable means are provided in connection with conventional moving coil transducers,to set up balancing forces for the diaphragm movement ranging from zero to a hundred percent of the combined centering forces of diaphragm suspension and enclosed air. According to another feature of the invention the said balancing forces are set up by adjustable electro-magnetic means to act upon the diaphragm of the transducer.

Further, according to the invention, mechanically connected to the transducer's diaphragm is fitted a former carrying two coils (or one coil split into two parts by a tap substantially at its physical center), wound in the same direction, else, would in opposite direction one to the other. These coils are adapted to adjustably operate as balancing or servo-coils, operating within a magnetic flux gap of the transducer frame. The balancing effect is achieved by a direct current adjustably set to a constant value and flowing in these balancing coils tending to move the diaphragm either forwards or backwards, dependent on the direction of the magnetic field then set up.

The balancing coils are so connected to the DC supply that the direction of energizing current flow is appropriate in the two coils, the resulting electromagnetic forces acting in opposition, thus to cause the diaphragm appropriately to become pulled towards that direction depending which of the two coils is most immersed in the magnetic flux gap.

Initially therefore the balancing coils are positioned on the coil former equi-distant from the center of the flux gap with the diaphragm at rest centered by its suspension. In this rest position the opposing forces of the DC energized balancing coils will balance each other, and cancel.

A transducer with the DC supply to its balancing coils set at maximum and operating in free air the diaphragm, when disturbed from its center-position, would be pulled towards its end-stops, either forwards or backwards. However, with the transducer mounted in the wall of a sealed enclosure the resulting compression or rarefaction of the enclosed airwould normally prevent this from happening, as in practice the force exerted by the enclosed air upon the diaphragm would exceed the force exerted by the DC-energized balancing coils, by at least a small amount.In order for this balance between the forces to exist for every momentary position of the diaphragm excursion, particularly with regard to the fact that the force with which the air is acting upon the diaphragm is greater when the enclosed air is compressed than when it is rarefied, the two balancing coils may be wound on the former with differing numbers of turns, and/or with varying pitch i.e.

density of turns. Likewise, the geometry of the flux gap may be optimised for this purpose. A further balancing adjustment may be effected by regulating the supply current, also thereby to adapt the transducer for mounting in enclosures of different size and air volumes. Consequently, when the balancing force acting through the coils on the mechanically connected diaphragm is adjusted to be equal to the centering force exerted on the diaphragm by the enclosed air, the transducer will behave as if the air volume was infinite, i.e. would exhibit its 'free air resonance'.

When the force on the balancing coils is adjusted so that it balances the combined centering force enclosed air and diaphagm suspension, the overall effect would bethatthe diaphragm would remain in any position within its movement range where it is placed, and the resonance frequency thus would be zero. Overbalancing, similarly a leak in the transducer's enciosure, would then result in the diaphragm flipping to, and remaining at either of its end-stops.

Therefore, the adjustment of the direct current for the balancing coils will result in the resonance frequency of the enclosed transducer to be set at any point between zero and the 'natural resonance frequency' of the system, i.e. when the balancing force is removed altogether, regardless of the air volume of the enclosure, allowing even for this enclosure to be tiny, i.e. where a sealed transducer chasis itself is substituting the enclosure. In practice however the DC current value would be set to allow for the lowest sound frequency the transducer is designed to handle.

In order to prevent or at least minimize 'bottoming' i.e. excessive excursion of the diaphragm when overdriven, the length of the coils along their axis can be limited. Then the end of the coil will move out of the magnetic flux and thereby reduce the balancing force even before the diaphragm can reach its end-stops; alternatively the last few turns of the balancing coils may be wound in a direction opposite to the rest so that on overdriving the balancing force becomes in fact reversed into a diaphragm centering force, thus, damping any excessive excursion of the diaphragm. Incidentally the same features may be applied to the design of the voice coil proper, and so add to the damping of any overdrive.

Examples of embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figures 1,2,3 and 4 show exemplary embodiments of moving coil and permanent magnet arrangements modified in accordance with the invention.

In figure 1, a moving coil type loudspeaker comprises a sealed enclosure 1, serving also as the speaker chassis, a diaphragm 2 with a flexible surround 3 and its suspension 4, a permanent pot-type magnet 5 comprising two annular air-gaps 6a & 6b, this magnet rigidly secured to the enclosure, and a coil former 7 on which is wound both the voice coil 8, and two balancing coils 9a & 9b. The coil former is rigidly secured to the diaphragm, and is axially movable in the magnet's air-gaps. The voice coil is connected in known manner to an amplifier.

The two balancing coils are connected to a DC source with such polarity that the forces produced when the diaphragm is moved off-center by the voice coil, act on each coil in the direction of the arrows shown. Thus, when, for instance, the diaphragm is drawn towards the enclosure, the coil 9b would enter deeper into the air-gap and produce a servo-force to the right, i.e. assist the voice coil in overcoming the resistance to compression of the enclosed air, and so balancing the force exerted by the enclosed air re-acting upon the diaphragm.

In the example Figure 2, the coil former loins exteded to the front of the diaphragm 11. In this arrangement, the pot magnet 12 and voice coil 13 also operate in the conventional way. In front of the diaphragm 11 is secured another pot magnet 14to the enclosure 15 by a spider 16, co-axially to the magnet 12. The forward part of the coil former 10 is freely axially movable inside the air gap 18 and carries the balance coils 17a & 17b. Operation of this embodiment is identical to that of Figure 1,the difference being that a separate magnet and flux gap is used for the two balancing coils.

In Figure 3 only one pot-magnet is used, and the main feature is that both voice coil and balancing coils share the same air-gap. In this example voice coil and balancing coils are wound in layers, or the balancing coils straddling the voice coil, with most of the winding of the balancing coils disposed at each end of the voice coil.

According to Figure 4, also only one pot magnet is used with DC energized balancing coils only, which are however connected to a DC supply having means to superimpose audio frequency onto the direct current. These means could be the two secondary windings of an audio transformer, connected with due regard to phase and polarity.

In these examplary embodiments, the magnet is shown as one pot; it may however be made up of a ring magnet with suitable pole pieces. As alternative too, the placing of the voice coil and balancing coils of Figure 1 may be reversed, i.e. the voice coil 8 shown in the gap 6a at the rear of the magnet may instead be placed in the front gap 6b, and the balancing coils at the rear; likewise in Figure 2 the placing of both voice coil and balancing coils may be reversed, provided of course that the respective flux gaps are optimised.

As the balancing force required of that coil which is balancing the cycle during which the enclosed air is rarefied is less than that of the coil balancing the compression cycle, it may be would with less turns than the other coil. Alternatively it may be connected to the DC supply via a resistance, thereby appropriately lowering the energising current passing through it.

The DC supply to the balancing coils may be switched off automatically after a short time delay, for instance by sensing the absence of amplifier background noise; also a cut-out may be provided to switch off the supply in the event of a fault condition, e.g. in case of the diaphragm hitting its end-stops.

Any ripple voltage remaining on the DC supply to the balancing coils may be fed out-of-phase into the voice coil or the audio amplifier for the purpose of humbucking.

In another embodiment of the invention the balancing coils when they are wound on the former with regularturns, the currentforthe balancing of the force exerted upon the diaphragm by the enclosed air is obtained from a tracking amplifier, the output of which is to be controlled by a sensor which senses the direction of positional movement of the diaphragm. This sensor may be of known type sensing the momentary pressure of the enclosed air in the sealed enclosure, or else, be of the optoelectronic type, in which the light from a light source falling on a light-sensitive resistor, photodiode or photo transistor is modulated, i.e. changed in intensity, using the obstruction or transmission by a contoured screen fitted to, or being part of the diaphragm.Alternatively a mirror attached to the diaphragm or the coil former may be used, its deflection to control the amount of light falling on either of two photo-sensors according to whether the diaphragm is forwards or rearwards of its center.

Adjustment of effective resonance frequency would be done by adjusting the gain, and thereby the output current, of the amplifier. This facility, by its effect on the resonance frequency of the system, may be used to compensate also for acoustical deficiency of the listening room, or for other special effects.

Moreover, said opto-electronic sensor, i.e. light sensitive resistor(s) may be made part of the DC supply circuit of the balancing coils directly to control the amount of current through the balancing coils, thus obviating the need for an amplifier. Any combination of the sensing and DC supply embodiments described may be used. In addition, the output of the (phto)sensor cam be used for known motion-feedback purposes, to superimpose certain correction signals onto the audio signal, or to vary automatically the balancing currentforthe balancing coils.

The invention furthermore may also be applied to passive units, such as are often used as auxiliary bass radiators, driven by means of air movement or pressure changes from the rear-facing side of the diaphragm of the active, i.e. electronically driven, transducer. When used as such a passive radiator, its voice coil may be used to generate a signal, for feedback into the audio chain, or a voice coil and its magnet can be omitted altogether, so that for this passive radiator, only balancing coils need be fitted with their adjustable supply. The passive radiator can now be adjusted to contribute acoustic output mainly at particular frequencies.

Moreover, a passive transducer, with its back closing its own box, or closing a sealed compartment of the enclosure for the active transducer, may according to the invention be fitted in such a way that the front of the passive transducer, and the rearward facing diaphragm of the active transducer are linked by an air passage.

When a signal is applied to the active transducer causing its diaphragm to move, say, backwards into the enclosure, the diaphragm of the linked passive transducer will be pushed backwards as well by the movement of the thus actively displaced air into its own box or compartment. By this passive diaphragm's backward movement, freely due to its balancing system, the space between the two transducers remains substantially of the same volume, without rise in pressure, thus resulting in the removal of back-pressure, i.e. restoring force, upon the active transducer, so that it can behave as if operating in free air, or, at any rate, in a much larger own enclosure, since the air behind it is merely displaced and not compressed.

Claims

1. An electro-dynamic acoustic transducer with a vibratory diaphragm, and a voice coil therefor, wherein there are provided means to oppose the centering forces acting upon said diaphragm by the mechanical suspension and enclosed air when said transducer is mounted in the wall of a sealed enclosure, said means including two coils, or one coil subdivided into two coils by a tap halfway along its length means for providing magnetic flux cutting said coils, and mechanical means connecting the said coils to one another and to said diaphragm the coils being so arranged that when an adjustable direct current is passed respectively through the said coils, electro-magnetic forces are produced which act respectively upon the said coils and are balanced only when the diaphragm is at its mid-position and the centering forces at zero.

2. A transducer according to claim 1, wherein the said magnetic flux is provided by a permanent magnet.

3. A transducer according to claim 1, wherein the said magnetic flux is provided by an electro-magnet.

4. A transducer according to claim 1-3, wherein the said flux is inhomogeneously distributed along the air gap for the coils.

5. A transducer according to claim 1, wherein the said coils are wound in the same direction.

6. A transducer according to claim 1, wherein the said coils are wound in opposite direction to one another.

7. A transducer according to claim 1, wherein the said coils are wound with non-uniform density of winding.

8. A transducer according to claim 1, wherein the said coils have a number of end-turns wound in opposite direction to the rest of the winding.

9. A transducer according to claim 1,wherein more than one vibratory motor system comprising said means is mechanically connected to the diaphragm, acting in unison upon said diaphragm, said coils being connected to the DC supply in series, parallel, or series-parallel connection, and provided individually with magnetic flux-producing means.

10. Atransducer according to claim 1, wherein the diaphragm is also mechanically connected to more than one voice coil, through which, in operation, audio-frequency currents are passed.

11. A transducer according to claim 1, wherein the said coils are co-axial to a voice coil, movable in separate co-axial gaps, and with separate or shared flux producing means.

12. Atransducer according to claim 1, and lo, wherein the voice coil has a number of end-turns wound in opposite direction to the rest of its winding.

13. Atransducer according to claim 1, wherein the coils of the means to oppose the centering forces are also operating coils of the transducer, and audio-frequency currents are superimposed upon said direct currents to produce out-of-balance forces upon the said coils.

14. Atransducer according to claim 1, wherein the said coils are wound as a layer on, under, interlaced with, or straddling a voice coil, all coils operating in the same magnetic flux gap.

15. Atransducer according to claim 1, 13, & 14, wherein more than one such motor-system is mechanically connected to the diaphragm.

16. A transducer according to claim 1,wherein the direct current for said coils is derived in a steady or pulsed form.

17. A transducer according to claim 1, wherein said coils are fed by direct current via pre-set, adjustable means.

18. Atransducer according to claim 1, wherein the adjustment is automatically effected by signals from a sensor of air pressure, or opto-electronic diaphragm position sensing means.

19. A transducer according to claim 1, wherein the residual or other ripples in the direct current supply for the coils is fed out-of-phase into the voice coil or audio amplifier.

20. A loudspeaker, comprising a transducer according to claim 1, wherein the diaphragm of said transducer is driven by the air displacement or pressure waves of a second transducer, linked to it via an air passage.

21. A loudspeaker, comprising a transducer according to claim 1 to 20, wherein a voice coil of the passive transducer is used to generate feedback signals for the control of the amplifier feeding the active transducer.

22. A transducer according to claim 1 & 20, wherein the voice coil and its flux producing means is omitted, and the diaphragm is activated in response to air displacement or pressure waves of a second transducer, or other sound source.

23. Atransducer according to any of the preceding claims, with timed switching means sensing the presence or absence of background noise or audio output of the amplifier, to switch-on or off said coils' direct current supply.

24. Atransducer according to the preceding claims, with direct current cut-out, automatically to de-activatethe coils! supply upon excessive diaphragm displacement due to overdrive, or a fault condition.

25. An electro-dynamic acoustic transducer substantially as described herein before-with reference to Figure 1,2,3, or 4 of the accompanying drawings.